Make frame_test_helpers's BeginNavigation load synchronous in the same cases as real code

RenderFrameImpl::CommitSyncNavigation() runs a navigation synchronous
only if (1) the navigation will load as empty, and (2) it is the
first navigation in the frame. TestWebFrameClient::BeginNavigation
now follows the same rules.

Bug: 855189
Change-Id: Ic5858ab1d8617a359d44f5e8cbc034de5bc4bf34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1548050
Commit-Queue: Nate Chapin <japhet@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Auto-Submit: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651820}